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As demonstrated during the March 2011 severe nuclear accident in Fukushima, Japan, accumulation and subsequent detonation of hydrogen gas produced by an overheated nuclear core reacting with steam can breach a reactor's containment structures and result in widespread radioactive contamination. The U.S. Nuclear Regulatory Commission (NRC) has a checkered history when it comes to requiring measures that would effectively reduce the risk of hydrogen explosions in the event of a severe accident at a U.S. nuclear power plant. This regulatory lapse is rooted in the history of the development of commercial nuclear power in the United States, when the NRC's predecessor agency, the Atomic Energy Commission (AEC), had a dual mandate: both to promote and to regulate commercial nuclear power.

As a consequence of this internal conflict of interest, rather than consult independent scientific and technical institutions, the AEC entrusted two companies that designed nuclear reactors -- Westinghouse and General Electric (GE) -- with the mission of demonstrating that in a large-pipe-break
loss-of-coolant accident (LOCA), the emergency core-cooling systems for their respective reactor designs would in fact prevent overheating of the core, and hence prevent the generation of large quantities of explosive hydrogen gas.

In response to the partial meltdown of Three Mile Island Unit 2 in 1979, the NRC revised its regulations regarding the control of hydrogen in an effort to help prevent hydrogen explosions in severe nuclear accidents, issuing new operating requirements in 1981 and again in 1985. By contrast, after Fukushima Daiichi's three devastating hydrogen explosions, the NRC decided to relegate investigating severe accident hydrogen safety issues to the lowest-priority and least proactive stage (Tier 3) of its post-Fukushima Daiichi accident response. Hence, beyond ensuring reliable containment pressure relief vents are added to obsolescent Fukushima-type reactors, it could take many years, or even decades, before the U.S. nuclear industry implements further hydrogen control measures.

Multiple technical pathways exist for minimizing the risk of hydrogen explosions in severe nuclear accidents. However, in the aftermath of the Fukushima Daiichi accident, the NRC has merely declared that severe nuclear accidents are vanishingly rare events that can be either prevented or sharply limited in scope, thereby avoiding any significant buildup of hydrogen and attendant explosion risk. The reality,
however, is that merely waving a rhetorical magic wand over the problem of hydrogen explosion risk flies in the face of a number of unresolved safety issues, including:

experimental evidence that current reactor computer safety models do not accurately predict the onset of rapid hydrogen generation in severe nuclear accidents, and that they under-predict the rates of hydrogen generation that occur in such accidents;

an aging fleet of U.S. reactors that will increasingly operate beyond the 40-year term of their initial licenses while facing severe competitive pressures from other electricity generation technologies, creating a perilous tradeoff between economic viability and public safety;

the compromised ability of 40-year old containments to prevent hydrogen leakage (for example, at the seals of pipe and cable penetrations) under the elevated-pressure conditions that are expected to occur in severe accidents;

the apparent willingness of the NRC to accede to licensee requests to relax and defer requirements for periodic containment pressurization and leak rate testing; and

the lack of technical readiness of U.S. power reactor owners to detect and control dangerous concentrations of hydrogen in all the places where it could migrate and explode in a nuclear power plant.

We conclude that the NRC is failing to meet the statutory standard of "adequate protection" of the public against the hazard of hydrogen explosions in a severe reactor accident. Our reasons are set forth in more detail in the body of this report.